This invention relates to distributed amplifier circuits for operation at very high frequencies, for example in the 20 MHz to 50 GHz range.
Distributed amplifiers can achieve a very wide bandwidth, because they do not involve the use of resonant circuits. Such amplifiers comprise one or more field effect transistors connected between two microwave transmission lines; a "gate" transmission line to which the gate electrodes of the transistors are connected, and a "drain" transmission line to which the drain electrodes are connected. The source electrodes of the transistors are connected to a "ground" line, which is common to the two transmission lines and to a microwave signal input and a microwave signal output. The gate and drain transmission lines are essentially artificial lumped transmission lines with inductors provided in the end sections of the lines and between electrodes of successive transistors. The transistors are preferably GaAs MESFETs.
In use of the amplifier, a microwave signal is fed into the microwave signal input and a respective travelling wave passes along each of the gate and drain transmission lines. The travelling waves are amplified by the or each transistor in turn and with the phase constants of the lines equal and correctly defined the gain of the amplifier is substantially independent of the signal frequency. Typically a gain of some 6 dB can be obtained over a very wide frequency band.
Whilst high gain and large bandwidth are the basic requirements of a distributed amplifier, it is also very important that the "noise figure" or increase in noise generated by the amplifier is kept as low as possible.
A detailed analysis of these noise components generated in a distributed amplifier is given in a paper by the present inventor, published in IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-33, No. 6, June 1985, pages 460-466, entitled "The Intrinsic Noise Figure of the MESFET Distributed Amplifier".